An axial flow turbine, for example a steam turbine, comprises a casing and a rotor which is rotatably supported within the casing. The rotor comprises a shaft and a plurality of rotor blade rings which are attached behind one another to the shaft. During operation of the turbine working fluid is expanded progressively by the blade rings to bring about driving the shaft.
Each rotor blade ring is formed by a plurality of rotor blades being circumferentially arranged, wherein two adjacent rotor blades form a blade passage. The rotor blades are aerodynamically profiled such that, when the working fluid passes the blade passages, the flow is turned and thereby a circumferential force on the rotor blades is generated. The circumferential forces on each blade of the rotor blade ring effect turning the rotor thereby generating shaft power.
The rotor blades are fixed to the shaft and extend therefrom towards the casing. The lateral ends of the rotor blades at the casing are formed into blade tips, wherein at the blade tips the rotor blade ring is shrouded by a shroud. The shroud is fixed to the blade tips and spaced apart from the casing thereby forming a tip clearance. The height of the tip clearance is dimensioned such that during operation of the turbine it is prevented that the shroud scrubs at the casing. Due to the fact that static pressure of the flow upstream of the rotor blade ring is higher than static pressure of the flow downstream of the rotor blade ring, during operation of the turbine a leakage flow passes the tip clearance.
The main flow passes the blade passages for shaft power generation, whereas the leakage flow bypasses the rotor blade ring via the tip clearance. Therefore, the leakage flow does not participate to the shaft power generation and does not flow through the blade passage. Further, the leakage flow after being re-entrained into the main flow path interferes with the main flow. Therefore, the main flow is locally inhomogeneous resulting in a mismatched flow. Furthermore, the tip clearance flow mixes with the main flow and generates disadvantageous dissipation. As consequence of this, the presence of the tip clearance flow affects the turbine efficiency.
In particular in high pressure turbines with low aspect ratio blades, the loss caused by the tip clearance flow is significantly high compared with the total losses of the turbine.
A remedy to reduce this negative effect of the tip clearance flow on the aerodynamic efficiency of the turbine is to take measurements reducing the tip clearance flow. A measurement, for example, is to provide a labyrinth seal on the outer circumference of the shroud within the tip clearance in order to reduce the mass flow of the tip clearance flow. As an alternative, a sealing element is fixed at the casing in the tip clearance. For fixing the sealing element to the casing, in the casing a circumferential groove is provided into which the sealing element is mortised.
Each of the solutions reduces tip leakage but does not eliminate the flow. As a result there is a continuing need to address turbine efficiency losses resulting from blade tip leakage.